Liquid crystal display and method of driving same

ABSTRACT

There is disclosed a lightweight and small liquid crystal display which achieves low power consumption and in which the optical anisotropy of the liquid crystal material is compensated for in order to enhance the viewing angle characteristics and the response speed of the liquid crystal material. Display electrodes and a common electrode are formed on one of the substrates. The orientation of the liquid crystal material is of the HAN (hybrid alignment nematic) type. This compensates for the optical anisotropy of the liquid crystal material and improves the response speed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/218,436, filed Sep. 6, 2005, now allowed, which is a continuation ofU.S. application Ser. No. 09/686,653, filed Oct. 10, 2000, now U.S. Pat.No. 6,963,382, which is a continuation of U.S. application Ser. No.08/751,365, filed Nov. 18, 1996, now U.S. Pat. No. 6,160,600, whichclaims the benefit of a foreign priority application filed in Japan asSerial No. 07-323677 on Nov. 17, 1995, all of which are incorporated byreference.

FIELD OF THE INVENTION

The present invention relates to an active matrix liquid crystaldisplay.

BACKGROUND OF THE INVENTION

Since liquid crystal displays are lighter and more compact displaydevices than CRTs, they find extensive application in computers,electronic calculators, clocks, and watches. The principle of operationof liquid crystal displays depends on a change in an optical property,such as interference, scattering, diffraction, optical rotation, orabsorption, of a liquid crystal material. This change is caused by avariation in orientation of the liquid crystal molecules or a phasetransition in response to application of an external field such aselectric field or heat.

Generally, a liquid crystal display comprises a pair of substrates witha given spacing which is maintained at 1 to tens of micrometers. Aliquid crystal material is held between these substrates, thus forming aliquid crystal panel. At least one of the substrates has transparency.Electrodes are formed on both or one of the substrates. Using theseelectrodes, an electric field is applied to the liquid crystal materialto control the orientation of the liquid crystal molecules in eachdifferent pixel within the substrate plane, thus controlling the amountof light transmitted through the liquid crystal panel. In this way, adesired image is displayed. The liquid crystal display is constructeddifferently according to which is the above-described optical propertiesis utilized, i.e., depending on the mode of operation. For example,polarizing plates are mounted on the Outside of the liquid crystalpanel.

To date, twisted-nematic (TN) liquid crystal displays andsupertwisted-nematic (STN) liquid crystal displays enjoy wideacceptance. These kinds of liquid crystal displays make use of opticalproperties of liquid crystal materials such as optical rotation andinterference of birefringent light. Both kinds require polarizingplates.

Where an image is displayed by the above-described liquid crystaldisplay, numerous pixels must be controlled at the same time. For thispurpose, various methods have been proposed. Among them, active matrixdriving is used widely because it is a method capable of displayingimage with high information content and high image quality. Inparticular, nonlinear active devices such as diodes or transistors arearranged at pixels. The individual pixels are electrically isolated fromeach other. Interference with unwanted signals is prevented. Thus, highimage quality is accomplished. In this method, each pixel can beregarded as a capacitor to which an electrical switch is connected.Accordingly, the switches are turned on and off according to the need.As a result, electric charge can be made to go into and out of thepixels. If the switches are turned off, electric charge can be retainedin the pixels and so the pixels can retain memory.

Problems with the Prior Art Techniques

(1) Electric Power Consumed by Liquid Crystal Display

In any kind of liquid crystal display, the liquid crystal materialitself does not emit light. In order to have good visibility, a lightsource is incorporated in the equipment (transmissive type), or lightincident on the equipment from surroundings is utilized (reflectivetype).

In the case of the transmissive type, if the intensity of light emittedby the light source is increased, then a brighter display device can beaccomplished accordingly. However, this increases the power consumptionof the whole apparatus. Most of the electric power is consumed by thelight source in the transmissive type liquid crystal display. The ratioof the electric power consumed by the liquid crystal panel to theelectric power consumed by the light source=1:100-1:1000. Hence, lowerpower consumption can be effectively accomplished by reducing theelectric power expended by the light source. However, in both TN and STNtypes, it is customary to use two polarizing plates. This considerablylowers the transmittance of the liquid crystal panel. Therefore, inorder to accomplish a bright display, it is necessary to increase thebrightness of the light source. Since it is necessary to maintain acertain degree of brightness, a great reduction in the power consumptioncannot be expected unless a light source of extremely high emissionefficiency is employed.

On the other hand, in the case of the reflective type liquid crystaldisplay, any special light source is not present inside the displaydevice and so low power consumption and a reduction in size are enabled.Hence, it can be said that the reflective type is an ideal displaydevice. However, it makes use of light coming from the surroundings: Inorder to achieve a brighter display device with a small amount of light,it is necessary to utilize the light efficiently. Since the reflectiveTN and STN types use no light sources, the brightness of the displaydevices is reduced accordingly.

(2) Response Speed

As image with higher information content is displayed, it is requiredthat the response speed of the liquid crystal material be increased.However, the response time of the above-described TN liquid crystaldisplay is tens of milliseconds. The response time of the STN liquidcrystal display is on the order of 100 ms. At these response speeds,when the image created on the viewing screen moves across the screen,the image appears to tail off. In this way, the image quality is notgood. In order to improve this phenomenon, it is necessary to utilize aliquid crystal material of higher response speed or to establish a modeof operation that accomplishes a higher response.

(3) Viewing Angle Characteristics

In both TN and STN types, when the liquid crystal display is viewedobliquely, a decrease in contrast, inversion of intermediate color hues,and other undesirable phenomena are observed, because the state ofpolarization of light entering obliquely changes in the liquid crystallayer of the liquid crystal display. In view of these problems, a methodof dividing each pixel electrode into plural parts, a method ofproducing plural different states of orientation, and other methods havebeen proposed but no fundamental solution to the problems is availabletoday.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a liquid crystaldisplay which is lightweight, small in size, capable of displaying imageof high image quality, shows decreased electric power consumption, andhas improved response speed and improved viewing angle characteristics.

The above object is achieved in accordance with the teachings of theinvention by a liquid crystal display comprising: a first insulatingsubstrate having transparency; a second insulating substrate disposedopposite to said first insulating substrate and reflecting light; aliquid crystal material sandwiched between said first and secondsubstrates; first pixel electrodes formed on said first insulatingsubstrate; first conducting lines for applying electrical signals tosaid first pixel electrodes, said first conducting lines being formed onsaid first insulating substrate; second pixel electrodes formed on saidfirst insulating substrate and electrically insulated from said firstpixel electrodes and from said first conducting lines; and secondconducting lines for applying electrical signals to said second pixelelectrodes, said second conducting lines being formed on said firstinsulating substrate.

The above object is also achieved by a liquid crystal displaycomprising: a first insulating substrate having transparency; a secondinsulating substrate disposed opposite to said first insulatingsubstrate and reflecting light; a liquid crystal material sandwichedbetween said first and second substrates; first pixel electrodes formedon said first insulating substrate; first conducting lines for applyingelectrical signals to said first pixel electrodes, said first conductinglines being formed on said first insulating substrate; second pixelelectrodes formed on said first insulating substrate and electricallyinsulated from said first pixel electrodes and from said firstconducting lines; and second conducting lines for applying electricalsignals to said second pixel electrodes, said second conducting linesbeing formed on said first insulating substrate. The liquid crystalmaterial is oriented in a first way near the first insulating substrateand in a second way near the second insulating, substrate. The first wayis different from the second way.

The above object is also achieved by a liquid crystal displaycomprising: a first insulating substrate having transparency; a secondinsulating substrate disposed opposite to said first insulatingsubstrate and reflecting light; a liquid crystal material sandwichedbetween said first and second substrates; first pixel electrodes formedon said first insulating substrate; first conducting lines for applyingelectrical signals to said first pixel electrodes, said conducting linesbeing formed on said first insulating substrate; second pixel electrodesformed on said first insulating substrate and electrically insulatedfrom said pixel electrodes and from said first conducting lines; andsecond conducting lines for applying electrical signals to said secondpixel electrodes, said second conducting lines being formed on saidfirst insulating substrate. The liquid crystal material is orientedparallel or substantially parallel to the first insulating substratenear the first insulating substrate and oriented vertically orsubstantially vertically to the second insulating substrate near thesecond insulating substrate.

The above object is also achieved by a liquid crystal displaycomprising: a first insulating substrate having transparency; a secondinsulating substrate disposed opposite to said first insulatingsubstrate and reflecting light; a liquid crystal material, sandwichedbetween said first and second substrates; first pixel electrodes formedon said first insulating substrate; first conducting lines for applyingelectrical signals to said first pixel electrodes, said conducting linesbeing formed on said first insulating substrate; second pixel electrodesformed on said first insulating substrate and electrically insulatedfrom said first pixel electrodes and from said first conducting lines;second conducting lines for applying electrical signals to said secondpixel electrodes, said second conducting lines being formed on saidfirst insulating substrate; and nonlinear devices connected with saidfirst or second pixel electrodes.

Furthermore, the present invention provides a method of driving a liquidcrystal display comprising a first insulating substrate havingtransparency, a second insulating substrate disposed opposite to saidfirst insulating substrate and reflecting light, a liquid crystalmaterial sandwiched between said first and second substrates, firstpixel electrodes formed on said first insulating substrate, firstconducting lines for applying electrical signals to said first pixelelectrodes, said first conducting lines being formed on said firstinsulating substrate, second pixel electrodes formed on, said firstinsulating substrate and electrically insulated from said first pixelelectrodes and from said first conducting lines, and second conductinglines for applying electrical signals to said second pixel electrodes,said second conducting lines being formed on said first insulatingsubstrate. This method consists of producing an electric field which hasa component parallel to the substrates and drives on the liquid crystalmaterial.

The structure of a liquid crystal display according to the presentinvention is shown in FIG. 1, where display electrodes 110 and a commonelectrode 111 are formed on a first substrate 101. A reflecting layer103 is formed on a second substrate 102. The liquid crystal displayfurther includes a biaxial film 104, a polarizing plate 105, and liquidcrystal molecules 106. The optical axis of the polarizing plate isindicated by the arrows 107. Liquid crystal molecules close to the firstsubstrate are oriented in one direction indicated by the arrows 108.Where the novel liquid crystal display is operated as an active matrixdisplay, nonlinear devices 109 (only one is shown) are formed. Theliquid crystal display further includes scanning (gate) lines 112 (onlyone is shown) and signal (source) lines 113 (only one is shown).

The first insulating substrate 101 is made from a material which hastransparency and shows some degree of resistance to an external force.For example, glass, quartz, or other inorganic material is used as thematerial of the first substrate 101. Where TFTs are fabricated on thesubstrate, non-alkali glass or quartz is used. Where a decrease in theweight of the liquid crystal panel is required, a film showing only asmall amount of birefringence such as PES (polyethylene sulfate) can beemployed.

The material of the second substrate 102 exhibits some degree ofresistance to an external force and reflects light. For instance, a thinfilm 103 of Al, Cr, or other material may be formed on the surface ofthe first substrate. It is also possible to use a substrate of Si.

In accordance with the present invention, in order to operate the liquidcrystal material, an electric field having a component parallel to thesubstrates is produced inside the liquid crystal cell. This electricfield will hereinafter be referred to as the lateral field. Theorientation of the liquid crystal molecules is controlled by thestrength of the lateral field. For this purpose, two kinds of electrodeswhich are electrically insulated from each other are formed on the firstsubstrate. That is, they are the display electrodes 110 and the commonelectrode 111. The two kinds of electrodes are made from a conductivematerial such as Al or Cr. If the electrodes are made from a materialhaving transparency such as ITO, then the aperture ratio of the pixelscan be enhanced. The fundamental structure of the electrodes is shown inFIG. 2. The display electrodes 110 and common electrode 111 havealternately protruding portions and alternately nested in each otherwith a given spacing. If necessary, a reflector formed on the secondsubstrate may be used as the electrodes.

The liquid crystal display according to the invention can use anoptically compensated birefringence (OCB) mode to improve the viewingangle. In the OCB mode, the refractive index is effectively uniform inthree dimensions. A fundamental structure for establishing the OCB modecomprises two polarizing plates together with a bend cell and a biaxialfilm inserted between them. This compensates for the refractive index inthree dimensions. The liquid crystal molecules in the bend cell areoriented in the manner described now. The major axis of the liquidcrystal molecules in the liquid crystal panel are parallel to thesubstrates near the substrates. The major axis rotate from one substrateto the other through 180° vertical to the substrates.

However, if the above-described structure is used as it is, then itfollows that two polarizing plates are used. This makes it impossible torealize a bright liquid crystal display. Accordingly, a reflectiveliquid crystal display according to the invention needs only onepolarizing plate. Specifically, in the OCB mode, the liquid crystalmolecules are grouped in two classes about the midway plane between thetwo substrates. These two groups of liquid crystal molecules have amirror-image relation to each other. If this midway plane is used as aturning point and the device is designed as a reflection type, then theliquid crystal molecules are oriented in exactly the same way as liquidcrystal molecules in a HAN-mode (HAN: hybrid alignment nematic) liquidcrystal display. In the HAN mode, liquid crystal molecules on the sideof one substrate are oriented vertically, while liquid crystal moleculeson the side of the other substrate are oriented horizontally. Theorientation of the liquid crystal molecules 106 when no electric fieldis applied is shown in FIG. 1. A nematic material having positive ornegative dielectric anisotropy is used as the liquid crystal material.To achieve the orientation, the substrate surface over which verticalorientation is assumed is processed with monobasic chromium complex, ora silane-coupling agent is applied in a manner not illustrated (notshown). Polyimide or the like is applied to the substrate surface overwhich horizontal orientation is assumed (not shown). The polyimide layeris rubbed in a well-known manner.

The liquid crystal material may be activated by a multiplexing method oractive matrix method. In the multiplex method, only two kinds ofelectrodes, i.e., display electrodes and common electrode, are requiredto be formed on the first substrate. However, in the active matrixmethod, a switching device consisting of a nonlinear device 109 such asa thin-film transistor (TFT) or diode must be added to each pixel. Theactive layer of each TFT can be made from amorphous silicon orpolysilicon (polycrystalline silicon). The nonlinear devices 109 areconnected with the display electrodes 110.

A more advanced structure has a peripheral driver circuit composed ofthin-film transistors which are integrated on a substrate. In thisstructure, pixel regions are integrated with the peripheral drivercircuit region on the substrate. Therefore, it is easier to exploit theliquid crystal panel.

To improve the display characteristics, a black matrix 328 (FIG. 3(F))is formed on those portions (i.e., conductive interconnects, nonlineardevice portions, and peripheral driver circuit) which are not associatedwith image display. The black matrix 328 is made of a metal such as Cror made of a transparent substance in which a black substance isdispersed to prevent contrast deterioration due to irregular reflectioninside the liquid crystal display. An inorganic material such as glassor quartz or an organic material such as resins can be used as thetransparent substance. Because of ease of fabrication, resinousmaterials are preferably used. Acrylic-based materials can be used asthe resinous material. Carbon black or a pigment can be used as theblack substance.

The method of dispersing the black material in the resinous material canbe appropriately selected according to the used black substance. Thematerials can be stirred by a stirrer, by ball milling, or with threerolls. Where the black material is dispersed, if a small amount ofsurfactant or other dispersion-assisting agent is added, thedispersibility of the black material can be enhanced. In order tostabilize the dispersion and to make the black matrix layer thinner, thediameter of the particles of the black material is preferably less than1 μm.

The black material can be formed on the TFT substrate by the same methodas used to form a resist pattern by a conventional photolithographicprocess. First, an organic solution in which the black substance isdispersed is applied to the TFT substrate by spin-coating orprint-coating techniques. The layer is patterned by a well-knownphotolithographic method. The layer is subsequently postbaked at about200° C.

In the HAN mode described above, color control is possible bycontrolling the voltage applied to the liquid crystal material.Therefore, color filters which have been used in the prior art liquidcrystal display are dispensed with.

The layers on the substrates are oriented in this way. The substratesare so placed that their surfaces having the oriented layers, TFTs, andtransparent electrode are located opposite to each other. A liquidcrystal material is held between the opposite substrates. Spacers aredispersed over the substrates to maintain the substrate spacing uniform.The used spacers have a diameter of 1 to 10 μm. The two substrates arebonded together with an epoxy adhesive or the like. The adhesive patternis formed outside the substrates so that the pixel regions andperipheral driver circuit region are located inside.

In the novel liquid crystal display, display electrodes and a commonelectrode are formed on one substrate. An electric field parallel to thesubstrates drives the liquid crystal molecules. The liquid crystalmolecules are made to assume the HAN orientation. This compensates forthe optical anisotropy of the liquid crystal material and improves theviewing angle characteristics. Furthermore, the HAN orientation giveshigher response speed than conventional TN and STN orientations. Where areflector is formed on the other substrate, a reflective liquid crystaldisplay can be built. Since it is not necessary to incorporate a lightsource in the display device, lower power consumption can beaccomplished.

Other objects and features of the invention will appear in the course ofthe description thereof, which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic exploded perspective view of a liquid crystaldisplay according to the present invention;

FIG. 2 is a fragmentary top view of an electrode structure included inthe liquid crystal display shown in FIG. 1;

FIGS. 3(A)-3(F) are cross-sectional views of pixel TFTs and a peripheraldriver circuit included in a liquid crystal display according to theinvention; and

FIG. 4 is a graph illustrating the transmission-voltage characteristicsof Embodiment 3 of liquid crystal display according to the invention.

DETAILED DESCRIPTION OF THE INVENTION Embodiment 1

The present embodiment is a liquid crystal display making use of amultiplexing drive method. First, a glass substrate 101 was prepared asa first insulating substrate. An ITO film having a thickness of 1200 Åwas formed and patterned into display electrodes. Then; an insulatingfilm of SiN was grown on the ITO film to a thickness of 1000 Å. AnotherITO film was formed on this insulating film to a thickness of 1200 Å andpatterned to form a common electrode. The structure of the displayelectrodes and common electrode is shown in FIG. 2. The displayelectrodes, indicated by 110, took a comb-like form. The displayelectrodes 110 had horizontally extending portions and verticallyextending portions. The vertically extending portions had a width,indicated by 203, of 10 μm. Also, the horizontally extending portionshad a width, indicated by 204, of 10 μm. The common electrode 111 hadhorizontally extending portions and vertically extending portions. Thevertically extending portions had a width, indicated by 205, of 10 μm.Also, the horizontally extending portions had a width, indicated by 206,of 10 μm. The length, 207, of the portions which are alternately nestedin each other was 60 μm. The spacing 208 between the display and commonelectrodes was 5 μm. A film of Cr was formed on a second insulatingsubstrate 102 to a thickness of 1200 Å. Reflecting function was impartedto the second insulating substrate.

In the present embodiment, the liquid crystal molecules were made toassume HAN orientation. For this purpose, an orientation film (notshown) was formed on each of the first substrate 101 and secondsubstrate 102. A layer of polyimide was formed on the first substrate101 by well-known spin-coating or DIP techniques. In order to orient theliquid crystal molecules parallel to the substrates, the polyimide filmon the first substrate 101 was rubbed in a direction indicated by thearrows 108 in portions corresponding to the teeth of the display andcommon electrodes. A silane-coupling agent was applied to the secondsubstrate 102. As a result, the liquid crystal molecules on the surfaceof the second substrate were oriented vertically.

The first substrate 101 and the second substrate 102 formed in this waywere stacked on top of each other, thus forming a liquid crystal panel.Spherical spacers (not shown) having a diameter of 3 μm were sandwichedbetween the two substrates to make the substrate spacing uniform overthe whole panel plane. An epoxy adhesive was used to bond together thesubstrates and to seal the panel. The patterned sealing adhesive wasapplied to a region (not shown) surrounding the pixel regions and theperipheral driver circuit region. The substrates were cut into a desiredshape. A liquid crystal material having positive dielectric anisotropywas then injected between the substrates. Nematic liquid crystalZLI-2293 (Δ∈=+10 at 1 kHz and at 20° C.) was used as the liquid crystalmaterial.

Then, a biaxial film 104 and a polarizing plate 105 were successivelystuck on the first substrate. The direction 107 of the optical axis ofthe polarizing plate made an angle of 45° to the rubbing direction.

This liquid crystal display was operated at a voltage of 3 V. An imagedisplay could be provided at a contrast of 100, a response speed of 2ms, and a wide viewing angle.

Embodiment 2

A method of fabricating a substrate for use in a liquid crystal displayutilizing the active matrix circuit of the present embodiment is nextdescribed.

A process sequence for obtaining a monolithic active matrix circuit ofthe present embodiment is described below by referring to FIGS.3(A)-3(F). This process sequence makes use of a low-temperaturepolysilicon process. The left sides of FIGS. 3(A)-3(F) show steps formanufacturing TFTs forming a driver circuit. The right sides show stepsfor manufacturing TFTs forming the active matrix circuit.

First, a glass substrate 301 was prepared as the first insulatingsubstrate. A silicon oxide film was formed as a buffer oxide film 302 onthe glass substrate 301 to a thickness of 1000 to 3000 Å by sputteringtechniques or plasma CVD in an oxygen ambient.

Then, an amorphous silicon film was formed to a thickness of 300 to 1500Å, preferably 500 to 1000 Å, by plasma CVD or LPCVD. Thermal annealingwas carried out above 500° C., preferably at a temperature of 500-600°C., to crystallize the silicon film or to enhance the crystallinity.Thereafter, photo-annealing might be effected, using a laser or thelike, to enhance the crystallinity further. During crystallizationutilizing the thermal annealing, a catalytic element such as nickel forpromoting the crystallization of silicon may be added, as described indetail in Japanese Patent Laid-Open Nos. Hei 6-244103 and Hei 6-244104.

Subsequently, the silicon film was etched to form islands of activelayers 303, 304 forming TFTs of the driver circuit and islands of anactive layer 305 forming TFTs of the matrix circuit (pixel TFTs). Theactive layer 303 was used to form P-channel TFTs, while the active layer304 was used to form N-channel TFTs. Then, a gate insulating film 306 ofsilicon oxide having a thickness of 500 to 2000 Å was formed bysputtering in an oxygen ambient. Instead, plasma CVD could be used toform the gate insulating film, in which case nitrous oxide (nitrogenmonoxide) (N₂O) or mixture of oxygen (O₂) and monosilane (SiH₄) wasadvantageously used as a gaseous raw material.

Then, an aluminum film having a thickness of 2000 to 6000 Å was formedby sputtering techniques over the whole surface of the substrate.Silicon, scandium, palladium, or other element may be added to thealuminum to prevent generation of hillocks during thermal processesconducted later. The aluminum film was etched into gate electrodes 307,308, and 309 (FIG. 3(A)).

Using these gate electrodes 307, 308, and 309 as a mask, phosphorus ionswere introduced into every island of the active layers 303-305 by aself-aligned ion implantation process. At this time, phosphine (PH₃) wasused as a dopant gas. The dose was 1×10¹² to 5×10¹³ atoms/cm². As aresult, weak N-type regions 310, 311, and 312 were formed (FIG. 3(B)).

Then, a photoresist mask 313 was formed over the active layer 303forming the P-channel TFTs. Another photoresist mask 314 was formedparallel to the gate electrode 309 and over those portions of the activelayer 305 forming the pixel TFTs which terminated in locations spaced 3μm from the end of the gate electrode 309.

Again, phosphorus ions were introduced by an ion implantation process,using phosphine as a dopant gas. The dose was 1×10¹⁴ to 5×10¹⁵atoms/cm². As a result, strong N-type regions (source and drain) 315,316 were created. During this implantation step, no phosphorus ions wereintroduced into that region 317 of the weak N-type region 312 of thepixel TFT active layer 305 which was coated with the mask 314. Hence,this region 317 remained lightly doped N-type (FIG. 3(C)).

Then, the active layers 304 and 305 for the N-channel TFTs were coatedwith a photoresist mask 318. Boron ions were introduced into the islands303 by ion implantation technology, using diborane (B₂H₆) as a dopantgas. The dose was 5×10¹⁴ to 8×10¹⁵ ions/cm². Since the dose of boron wasin excess of the dose of phosphorus (FIG. 5(C)), the previously formedweak N-type region 310 was converted into a strong P-type region 319.

As a result of these ion implantation steps, strong N-type regions(source/drain) 315, 316, strong P-type region (source/drain) 319, andweak N-type region (lightly doped region) 317 were formed. In thepresent embodiment, the width x of the lightly doped region 317 wasabout 3 μm (FIG. 3(D)).

Thereafter, a thermal annealing step was carried out at 450 to 850° C.for 0.5 to 3 hours to heal the damage caused by the ion implantation, toactivate the dopants and to recover the crystallinity of the silicon.Then, a silicon oxide film was formed as an interlayer insulator 320 toa thickness of 3000 to 6000 Å by plasma CVD over the whole surface. Thisinterlayer insulator can be a silicon nitride film or a multilayer filmconsisting of silicon oxide and silicon nitride. The interlayerinsulator film 320 was etched by wet etching techniques or dry etchingtechniques to form contact holes in the source/drain regions.

Subsequently, an aluminum film or a multilayer film of titanium andaluminum was formed to a thickness of 2000 to 6000 Å by sputteringtechniques. This film was etched to form electrodes and conductiveinterconnects 321, 322, 323 of the peripheral circuit and electrodes andconductive interconnects 324, 325 of the pixel TFTs. This was followedby formation of a passivation film 326 having a thickness of 1000 to3000 Å. The passivation film was made of silicon nitride. Thepassivation film was etched to create an interlayer insulator film. AnITO film 327 for forming display electrodes was formed to a thickness of1200 Å on the interlayer insulator film and patterned in exactly thesame way as the display electrodes of Embodiment 1.

Then, a silicon nitride film 329 was formed as a passivation film to athickness of 1000 to 3000 Å by plasma CVD. An ITO film (not shown)having a thickness of 1200 Å was formed as a common electrode on thepassivation film. This film was patterned into the same stripes as thecommon electrode of Embodiment 1 (FIG. 3(E)).

A black matrix 328 was formed on the laminate. In this embodiment, theblack matrix 328 was at the highest level. The ITO layer and the blackmatrix may be interchanged in level. As the black matrix material,carbon black particles having an average diameter of 1000 Å weredispersed in an acrylic-based resinous material. The obtained solutionwas applied by spin-coating or printing techniques. Then, a prebakeoperation was performed at 100° C. for 2 minutes. The laminate waspatterned by a well-known photolithography method. At this time, theintensity of the used ultraviolet irradiation was greater (more than 20mW/cm²) than the intensity used in normal patterning processes.Alternatively, after the formation of the black matrix, anoxygen-shielding film was made from PVA (polyvinyl alcohol) or the like.A developer solution obtained by dissolving 2.36% by weight of TMAH inwater was used for development. Consequently, a black matrix having athickness of 1 μm could be formed over all of the peripheral drivercircuit, the pixel TFTs, and gate and source lines. The aperture ratioof the pixel regions was 60%.

In the present embodiment, the liquid crystal molecules were made toassume HAN orientation. For this purpose, an orientation film was formedon each of the aforementioned first and second substrates. A polyimidefilm was formed on the first substrate by well-known spin-coating or DIPtechniques. To orient the liquid crystal molecules parallel to thesubstrates, the polyimide film on the TFT substrate was rubbed in adirection parallel to those portions which corresponded to the teeth ofthe display and common electrodes. The silane-coupling agent was appliedto the second substrate. As a result, the liquid crystal molecules onthe surface of the color filter substrate were oriented vertically.

The TFT substrate and counter substrate fabricated in this way werestacked on top of each other, thus forming a liquid crystal panel. Thespherical spacers having a diameter of 3 μm were interposed between thetwo substrates to make the substrate spacing uniform over the wholepanel plane. An epoxy adhesive was used to bond together the substratesand to seal the panel. The patterned sealing adhesive surrounded thepixel regions and the peripheral driver circuit region. The substrateswere cut into a desired shape. A liquid crystal material was theninjected between the substrates. Nematic liquid crystal ZLI-2293 wasused as the liquid crystal material.

Then, a biaxial film 104 and a polarizing plate 105 were successivelystuck on the first substrate. The direction 107 of the optical axis ofthe polarizing plate made an angle of 45° to the rubbing direction.

This liquid crystal display was operated at a voltage of 3 V. An imagedisplay could be provided at a contrast of 100, a response speed of 2ms, and a wide viewing angle.

Embodiment 3

In the present embodiment, a color display was provided using the liquidcrystal display of Embodiment 2. As the pixel voltage applied across theliquid crystal display was varied, the intensity of transmitted lighthaving a wavelength of 554.6 nm was varied. This is illustrated in FIG.4. As can be seen from the graph of FIG. 4, the transmission variedcontinuously with varying the voltage. No clear threshold value existed.Variations in color hues were observed. When no voltage was applied, thedevice exhibited a color of yellow-green. When a voltage of 0.5 V wasapplied, it exhibited a color of green. When a voltage of 0.9 V wasapplied, it exhibited a color of blue. When a voltage of 1.2 V wasapplied, it exhibited a color of red.

A color display was provided, by making use of this phenomenon and bycontrolling the pixel voltage applied across the present embodiment ofliquid crystal display. A multicolor display could be provided at anoperating voltage of 3 V with a wide viewing angle.

The configuration of the present invention permits fabrication of abright display device which needs no backlighting arrangement.Furthermore, the novel display device requires only one polarizingplate, unlike the prior art liquid crystal display. Consequently, apower consumption reduction can be accomplished. Furthermore, theoperating voltage decrease makes it possible to use batteries as a powersupply. This facilitates application to various portable electricalappliances.

What is claimed is:
 1. A method of driving a liquid crystal display device, said liquid crystal display device comprising: a first substrate; a thin film transistor formed over the first substrate and comprising a metal gate electrode; a transparent pixel electrode electrically connected to the thin film transistor; a transparent common electrode formed above the transparent pixel electrode; an insulating film between the transparent pixel electrode and the transparent common electrode; a second substrate being disposed opposite to the first substrate; a liquid crystal material between the first and second substrates wherein the transparent common electrode is located between the liquid crystal material and the insulating film; said method comprising applying a voltage between the transparent pixel electrode and the transparent common electrode to drive the liquid crystal material.
 2. The method according to claim 1, wherein the insulating film is a single layer.
 3. The method according to claim 1, wherein the insulating film comprises a silicon nitride film.
 4. The method according to claim 1, wherein the transparent pixel electrode comprises indium tin oxide.
 5. The method according to claim 1, wherein the transparent common electrode and the transparent pixel electrode do not overlap with each other.
 6. The method according to claim 1, wherein the liquid crystal display device is driven by an optically compensated birefringence mode.
 7. The method according to claim 1, wherein the metal gate electrode comprises aluminum.
 8. A method of driving a liquid crystal display device, said liquid crystal display device comprising: a first substrate; a thin film transistor formed over the first substrate and comprising a metal gate electrode and a channel region adjacent to the metal gate electrode, the channel region comprising polycrystalline silicon; a transparent pixel electrode electrically connected to the thin film transistor; a transparent common electrode formed above the transparent pixel electrode; an insulating film between the transparent pixel electrode and the transparent common electrode; a second substrate being disposed opposite to the first substrate; a liquid crystal material between the first and second substrates substrates wherein the transparent common electrode is located between the liquid crystal material and the insulating film; said method comprising applying a voltage between the transparent pixel electrode and the transparent common electrode to drive the liquid crystal material.
 9. The method according to claim 8, wherein the transparent pixel electrode comprises indium tin oxide.
 10. The method according to claim 8, wherein the transparent common electrode and the transparent pixel electrode do not overlap with each other.
 11. The method according to claim 8, wherein the liquid crystal display device is driven by an optically compensated birefringence mode.
 12. The method according to claim 8, wherein the metal gate electrode comprises aluminum.
 13. A method of driving a liquid crystal display device, said liquid crystal display device comprising: a first substrate; a thin film transistor formed over the first substrate and comprising a metal gate electrode; a transparent pixel electrode electrically connected to the thin film transistor; a transparent common electrode formed over the transparent pixel electrode; an insulating film between the transparent pixel electrode and the transparent common electrode; a second substrate being disposed opposite to the first substrate; a liquid crystal material between the first and second substrates wherein the transparent common electrode is located between the liquid crystal material and the insulating film; said method comprising applying an electric field produced between the transparent pixel electrode and the transparent common electrode to the liquid crystal material.
 14. The method according to claim 13, wherein the transparent pixel electrode comprises indium tin oxide.
 15. The method according to claim 13, wherein the transparent common electrode and the transparent pixel electrode do not overlap with each other.
 16. The method according to claim 13, wherein the liquid crystal display device is driven by an optically compensated birefringence mode.
 17. The method according to claim 13, wherein the metal gate electrode comprises aluminum. 